The invention concerns the field of the rectification of liquid mixtures of two components having different boiling points.
The separation by distillation of the components of a binary liquid mixture is a well known and tested method. As a general bibliographical reference on this subject, for example, the work of S. G. SHINSKEY "Distillation control for production and energy conservation", McGraw-Hill, USA (1977), may be cited and is herein incorporated by reference.
It would certainly be useful to improve conventional rectification for the purpose of saving energy by trying to obtain better thermodynamic yields.
Rectification columns have already been equipped with heat pumps using as sources the condensers and the column boilers. These operations heretofore were limited to applications wherein the boiler temperatures did not exceed approximately 80.degree. C., as the result of the temperature limitations imposed on the operation of heat pumps presently available on the market. Even when using working fluids providing higher performances with boiler temperatures exceeding 80.degree. C. and attaining for example 120.degree. C., only a small portion of existing columns may be thus equipped. Furthermore, the operations actually realized remain highly restricted in numbers, as the economics of their use do not permit rapid recovery of the investment. The performance of heat pumps (abbreviated PAC) are inadequate by reason of the limited thermodynamic yield of conventional materials. In the case under consideration, this yield is aggravated by the exchange temperature deviations due to the use of an intermediate heat transfer fluid between the "condenser-column"-"boiler-PAC" and "column boiler"-"PAC condenser".
The process according to the invention makes it possible both to avoid all temperature limitations and to employ the pumping of heat between the condenser and the boiler of the column with the best thermodynamic yield possible.
The invention takes advantage of a known process designated a "polytropic" process, which is employed in machines designated "polytropic". Such processes and machines are described particularly in French applications Nos. FR 74 114 38 (publication No. 23 07 227), FR 76 14 965 (publication No. 23 52 247) and FR 77 07 041, herein incorporated by reference.
Polytropic machines consist of a series of cells with staggered pressures/temperatures, wherein a working fluid circulates in the form of saturating vapor in contact with its liquid. Furthermore, there is or are present, at least in certain cells, one or several bundles of heating or cooling tubes, which expose the cells to heat transfer fluids introducing the heat of a producing source or extracting heat intended for a consuming zone. Finally, each cell is related to its neighbors, on the one hand, over the path of the vapor, by means of a compressor or a turbine, depending on whether the primary heat entering the process is available on the average at a high level of temperature or at a low level, the vapor ascending or descending the levels of pressures/temperatures thus involving a certain external work which may be designated the "work of transfer" and on the other hand, over the path of the liquid circulating in a direction inverse to that of the vapor and in an equal amount, through a calibrated orifice to descend the levels of pressures/temperatures or by means of a pump to ascend the said pressures/temperatures. It will be sufficient for those skilled in the art to refer to the descriptions of the abovecited patents in order to recognize the structure and functioning of such machines.
In the case wherein the heat transfer liquid introduces heat (it then circulates by traversing in series the stages in the direction of decreasing temperatures), the vapor of the working fluid is produced by the boiling of the liquid present in the cell, and, in the contrary case, the vapor of the working fluid condenses. Thus, the flow rates of the vapor and the liquid develop from stage to stage in accordance with the quantities of heat added or extracted as a function of the Q(T) law by which the addition or extraction of the heat are effected, i.e. as a function of the dimensions of the heat transfer bundles.
It is important to note that in principle, at the interface of two successive cells, the sum of the flow rates of the working fluid entering in the form of vapor or in the form of liquid is always equal to the sum of the flow rates of the same working fluid exciting in the form of liquid or the form of vapor, with the flows of the vapor and the liquid of the working fluid circulating in inverse directions within a stage always being equal.
It may be noted that the polytropic machines described in the above cited patents may be formed by four simple, elementary sequences, namely:
sequence of cooled compressors, used for a process of condensation with the absorption of work;
sequence of heated compressors, used in a process of boiling with the absorption of work;
sequence of cooled turbines used in a condensation process with work provided;
sequence of heated turbines, used in a boiling process with work provided.
All of these four elementary types of sequences comprise an open terminal stage whereby the liquid and vapor flows of the working fluid enter and exit, and a closed terminal stage wherein the working fluid is either completely vaporized, or completely condensed.
The table hereinafter indicates the side where the open stage may be found, the entries and exits of the working fluid in regard to the sequence considered, together with the direction of the heat transfer fluid.
______________________________________ Direction of the heat Type of Sequence Open Stage Working Fluid transfer fluid ______________________________________ Sequence of heated higher tem- liquid enters, decreasing compressors perature vapor exits temperatures Sequence of cooled lower tem- vapor enters, increasing compressors peratures liquid exits temperatures Sequence of heated lower tem- liquid enters, decreasing turbines peratures vapor exits temperatures Sequence of cooled higher tem- vapor enters, increasing turbines peratures liquid exits temperatures ______________________________________
In these systems, the heat transfer fluid may transverse several successive stages, or a single stage. In the extreme case, there may be a heat transfer circuit of a different nature per stage of a predetermined sequence.
It is also known that the operation of a polytropic machine may be generalized in the case wherein the liquid and vapor flow rates of the working fluid at the inlet of the open stage are different; in this case, the difference of the flows circulating in the two directions remains at a value it has at the inlet until the other terminal stage is reached, which is then traversed by a flow of the working fluid and thus is no longer a closed stage, it is said in such a case that one is in the presence of a "process open at both ends" or more simply an open process.
It is thus noted that the polytropic process that is the most general, has the following characteristics:
(1) It is multiple stage with regard to pressure and temperature.
(2) In each stage, the liquid and the vapor of the condensable working fluid are in contact and exchange both heat and material.
(3) The vapor of the working liquid circulates from stage to stage traversing rotating machines, thus involving work. The difference in temperature between two successive stages exists in principle only because of this single fact; the temperature rises from one stage to the other in the direction of the travel of the vapor if the rotating machine is a compressor, while it decreases if a turbine is involved.
(4) The liquid of the working fluid circulates from stage to stage in the direction inverse to the vapor; the liquid and vapor flows circulating between two stages have a difference which is reflected from stage to stage.
(5) Each stage may exchange heat with the outside.
The object of the present invention essentially is a process intended for the rectification of a binary mixture of two components A and B, A being more volatile, without the external addition of heat and providing only work, the latter being of a value close to the theoretical value necessary for the separation of the components A and B (designated the work of separation).
In the most general form, the object of the invention thus is a process for the rectification of a liquid mixture of two components A and B, A being the more volatile, wherein an open polytropic process using a condensable working fluid is utilized, said process comprising a plurality of stages, in each of which the liquid and the vapor of said working fluid are present, the flows of the vapor and the liquid of the working fluid circulating in inverse directions from one stage to the other, the difference berween the liquid and vapor flow rates of the working fluid circulating between two contiguous stages being conversed from stage to stage until the terminal stages; a process wherein, at the level of each stage, work and heat exchange with an external heat transfer fluid may be involved, said process being characterized in that the mixture of A+B to be rectified is used as the working fluid in the above-mentioned open polytropic process, said process being designated the principal process, with the terminal stages thereof being designated "first stage" and "last stage", so that one travels from the first to the last stage by following the direction of the travel of the vapor in said process, and comprising a pressure-temperature distribution such that in the first stage there is found only the component B in a practically pure state and in the last stage only a practically pure A component, in that in the stage wherein the liquid concentration is closest to that of the mixture, the charge to be rectified is introduced, it being a mixture composed of a flow .DELTA.m.sub.A of component A and .DELTA.m.sub.B of component B, previously brought to the pressure-temperature of said stage, designated the feed stage, the stages proceeding from the increasing direction from the feed stage to the last stage, being called the rectification stages and constituting together a rectification module, the difference of the vapor and liquid flows of the B component being essentially zero in the rectification module, while the difference of the vapor and liquid flows of the A component is equal to .DELTA.m.sub.A and is reproduced from stage to stage in the rectification module to the last stage, where a vapor flow .DELTA.m.sub.A is recovered, together with a supplemental vapor flow m.sub.A *, by that the same flow m.sub.A * is reinjected, after condensation, in the last stage to constitute the reflux of A, the stages proceeding in the decreasing direction from the feed stage to the first stage being designated the depletion stages and constituting together a depletion module, the difference of the vapor and liquid flows of the component A in the depletion module being essentially zero, while the difference of the vapor and liquid flows of the component B is equal to .DELTA.m.sub.B and is reproduced from stage to stage in the depletion module until the first stage, wherein a liquid flow .DELTA.m.sub.B, accompanied by a supplemental liquid flow m.sub.B *, is recovered, by that the same flow m.sub.B * is reinjected, after vaporization, in the first stage to constitute the reflux of B, with the vapor flow .DELTA.m.sub.A in the last stage and the liquid flow .DELTA.m.sub.B in the first stage constituting the production.